The present invention relates to a system in which a control apparatus communicates with a wireless communication terminal via a relay apparatus, and more particularly to a system in which the control apparatus and the relay apparatus are connected to each other via an optical transmission path.
Recently, wireless communication systems in which a relay station for communicating with a wireless communication terminal wirelessly is connected to a control apparatus via an optical transmission path have been used (for example, Japanese Laid-Open Patent Publication No. 9-233050).
FIG. 43 is a diagram showing a structure of a conventional wireless communication system described in Japanese Laid-Open Publication No. 9-233050. In the wireless communication system shown in FIG. 43, a control apparatus 19 converts a modulated signal into an optical signal and transmits the optical signal to the relay apparatus 29 via an optical transmission path 59. The relay apparatus 29 converts the optical signal transmitted from the control apparatus 19 into an electric signal by an optical-to-electric conversion section 95, and transmits the electric signal as a wireless signal to a wireless communication terminal 39 in the same area via a transmitting/receiving section 93 and an antenna section 92. The relay apparatus 29 receives the wireless signal transmitted from the wireless communication terminal 39 by the antenna section 92, converts the wireless signal into an optical signal by an electric-to-optical conversion section 94 via the transmitting/receiving section 93, and transmits the optical signal to the optical transmission path 59. In the conventional wireless communication system, communication between the relay apparatus and the wireless communication terminal is realized in this manner.
The conventional wireless communication system needs to fulfill the first through third requirements described below in order to realize a high communication quality.
The first requirement is that the levels of wireless signals received by the relay apparatus need to be kept within a predetermined range. The difference between the maximum level and the minimum level of a received wireless signal at which the received wireless signal can be effectively reproduced is referred to as a dynamic range. In the case where the level of a wireless signal received by the relay apparatus is too high, when the wireless signal is converted into an optical signal, the optical signal is distorted. By contrast, in the case where the level of a wireless signal received by the relay apparatus is too low, the wireless signal to be received and noise cannot be separated from each other. In order to realize high quality optical transmission of a wireless signal, the levels of the wireless signals received by the relay apparatus need to be kept within a predetermined range.
Next, when the relay apparatus amplifies a signal component to be transmitted to the wireless communication terminal, out-band frequency components with respect to the signal component to be transmitted are output due to the nonlinearity of the amplifier, together with the signal component to be transmitted. Out-band frequency components have adverse effects on other communication devices which communicate using the same frequency band as that of the out-band frequency components, electric devices existing in the vicinity of the wireless communication terminal, and the like. Therefore, the levels of the out-band frequency components with respect to the level of the wireless signal component to be transmitted by the relay apparatus need to be kept to a certain level or lower. Herein, the ratio between the level of the out-band frequency component and the level of the wireless signal component to be transmitted to the wireless communication terminal by the relay apparatus is referred to as a “leakage ratio”. Thus, the second requirement is that the leakage ratio of wireless signals needs to be kept to a certain level or lower.
Further, in order to allow the relay apparatus to normally communicate with the wireless communication terminal, the signal-to-jam ratio in the frequency band of a wireless signal transmitted or received (hereinafter, referred to as a “D/U (Desired/Undesired) ratio”) needs to be kept to a certain level or higher. The reason is that when the D/U ratio is too low, the relay apparatus cannot separate the signal to be received and noise from each other. Thus, the third requirement is that the D/U ratio of a wireless signal received by the relay apparatus needs to be kept to a certain level or higher.
Now, in order to solve the first problem, a wireless communication system shown in FIG. 44 is proposed (see the specification of Japanese Patent No. 2885143). FIG. 44 is a diagram showing a structure of a conventional wireless communication system described in the specification of Japanese Patent No. 2885143. In the wireless communication system shown in FIG. 44, a relay apparatus 28 and a control apparatus 18 are connected to each other via an optical fiber 58. The relay apparatus 28 receives a wireless signal transmitted from a wireless communication terminal (not shown) via antenna section 91. An amplifier 1 amplifies the signal received by the antenna section 91. The signal amplified by the amplifier 1 is divided by an optical divider 2 and frequency-converted by mixers 3a through 3d and synthesizers 4a through 4d. The frequency-converted signals pass through band-pass filters 5a through 5d having a one-wave passband, and then are amplified to a predetermined signal level by nonlinear amplifiers 6a through 6d. The amplified signals are combined by a combiner 7 and then converted into an optical signal by an electric-to-optical converter 8. The optical signal is transmitted to the control apparatus 18 via the optical fiber 58. In the control apparatus 18, an optical-to-electric converter 21 converts the optical signal transmitted from the optical fiber 58 into an electric signal. The electric signal is divided by a divider 11 and then frequency-converted by mixers 12a through 12d and oscillators 13a through 13d in order to be returned to have the original frequency band. Then, the frequency-converted signals are separated from one another by band-pass filters 14a through 14d having a one-wave passband. The separated signals are demodulated by demodulators 15a through 15d and output to the outside, or are detected by detectors 16a through 16d. The signals which are detected and output by the detectors 16a through 16d are converted into digital signals by A/D converters 17a through 17d and then stored in ROMs 18a through 18d. 
As described above, the conventional wireless communication system described in the specification of Japanese Patent No. 2885143 separates a received signal one wave by one wave using the relay apparatus 28 and adjusts the level of each separated signal by each of the nonlinear amplifiers 6a through 6d. Thus, by providing the nonlinear amplifiers 6a through 6d in the relay apparatus 28, the level of a wireless signal received by the relay apparatus can be kept within a predetermined dynamic range. In this manner, the first requirement can be fulfilled.
Next, in order to fulfill the second and third requirements, the IEEE802.11a Standard, for example, regulates the quality of a wireless signal transmitted and received by a relay apparatus and a wireless communication terminal. The IEEE802.11a Standard regulates that when the modulation system of the wireless signal is 64 QAM (Quadrature Amplitude Modulation) and jamming from other channels exists, the dynamic range of the wireless signal received by the relay apparatus must be about 32 dB at the maximum. Based on the IEEE802.11a Standard, the required D/U ratio is calculated to be about 22 dB or higher. Furthermore, ARIB STD-T71 regulates that the leakage ratio of a wireless signal to adjacent channels must be −25 dB or lower and that the leakage ratio to channels which are adjacent to the adjacent channels must be −40 dB or lower. By using a wireless signal fulfilling these regulations, the relay apparatus can normally communicate.
However, in the conventional wireless communication system described in the specification of Japanese Patent No. 2885143, the relay apparatus needs to have AGC (Automatic Gain Control) functions for controlling the gain such as, for example, the nonlinear amplifiers 6a through 6d in a number corresponding to the number of wireless channels, in order to keep the wireless signals within the dynamic range. This complicates the structure of the system.
Even in the case where a signal is transmitted to a control apparatus using a frequency-division multiplexing system also, there is a problem that the structure of the system is complicated. A relay apparatus, when receiving wireless signals from a plurality of wireless communication terminals simultaneously, cannot adjust the levels of the plurality of wireless signals at one time. In the wireless communication system described in the specification of Japanese Patent No. 2885143, the relay apparatus first separates the frequency-converted signal into a number of signals corresponding to the number of the channels and adjusts the levels of the signals to be the same, and then performs multiplexing. Therefore, the relay apparatus needs to include many components such as a divider, and mixers, synthesizers, band-pass filters, and nonlinear amplifiers in a number corresponding to the number of channels of wireless signals. This complicates the structure of the relay apparatus and makes it difficult to reduce the size of the relay apparatus.
In addition, in the conventional wireless communication system described in the specification of Japanese Patent No. 2885143, the relay apparatus and the control apparatus need to have an oscillator. As a result, there is a problem that the system having such a structure is expensive.
As described above, the conventional wireless communication system described in the specification of Japanese Patent No. 2885143, although being capable of fulfilling the first requirement, has a problem of complicating the structure thereof.
Moreover, the IEEE802.11a Standard does not provide any regulation for the case where the relay apparatus uses wireless signals of a plurality of channels. Therefore, the regulations on a wireless signal provided by the IEEE802.11a Standard cannot be applied as they are to a system using a plurality of channels. The reasons for this will be described below.
In the case where communication is performed using signals of a plurality of channels, channels are assigned different frequencies, respectively. FIG. 45 is a diagram showing spectra of wireless LAN signals in conformity to the IEEE802.11a Standard, which are transmitted from first and second wireless communication terminals which use two adjacent channels. The solid line represents the spectrum of signal a, and the dashed line represents the spectrum of signal b.
Signal a transmitted from the first wireless communication terminal and signal b transmitted from the second wireless communication terminal belong to channels adjacent to each other. Hereinafter, a case where signal a is jammed by a signal component leaking from signal b will be described.
Signal a has a signal component 1001a, a signal leakage component 1002a, and a signal leakage component 1003a. The signal component 1001a is a component of signal a which is to be received by the relay apparatus. The bandwidth of the signal component 1001a is about 20 MHz. The signal leakage component 1002a is a component leaking to the channel closest to that of the signal component 1001a (hereinafter, referred to as an “adjacent channel”) The signal leakage component 1003a is an out-band frequency component with respect to signal a and is a component leaking to the channel second closest to that of the signal component 1001a (hereinafter, referred to as a “second adjacent channel”). In the case where an out-band frequency with respect to a signal overlaps the frequency of the channel adjacent to the signal, the out-band frequency component leaks to the adjacent channel. Here, for the sake of simplicity, the following description will be given with the spectra of the signals being assumed to be at the same level.
The IEEE802.11a Standard regulates that in a wireless communication system in which the modulation system of a wireless signal is 64 QAM and jamming from other channels exists, the dynamic range of the wireless signal received by the relay apparatus must be about 32 dB at the maximum. Based on the IEEE802.11a Standard, the required D/U ratio is calculated to be about 22 dB or higher.
A leakage ratio 1004 is the ratio between the signal leakage component 1002a and the signal component 1001a, and is regulated to be −25 dB or lower. When the signal level is represented by a logarithm, the logarithm of the leakage ratio 1004 is represented by a difference between the logarithm of the level of the signal leakage component 1002a and the logarithm of the level of the signal component 1001a. The following description in this specification will be given with the leakage ratio being assumed to be represented by a logarithm. A leakage ratio 1005 is a difference between the level of the signal leakage component 1003a and the level of the signal component 1001a, and is regulated to be −40 dB or lower.
Signal b has a signal component 1001b, a signal leakage component 1002b, and a signal leakage component 1003b. The signal component 1001b is a component of signal b which is to be received by the relay apparatus. The signal leakage component 1002b is a component leaking to the adjacent channel to the signal component 1001b. The signal leakage component 1003b is a component leaking to the second adjacent channel to the signal component 1001b. 
A D/U ratio 1010 is a difference between the level of the signal component 1001a and the level of the signal leakage component 1002b. The signal leakage component 1002b leaks to signal a in the adjacent channel. Accordingly, in order to allow the relay apparatus to convert only signal a into an optical signal without being jammed by the leakage from the adjacent channel, the difference between the level of the signal component 1001a and the level of the signal leakage component 1002b, i.e., the D/U ratio 1010, must be 22 dB or higher.
The level of a wireless signal received by the relay apparatus depends on the distance between the relay apparatus and the wireless communication terminal. In other words, as the distance between the relay apparatus and the wireless communication terminal is longer, the level of the wireless signal received by the antenna of the relay apparatus is lower.
Accordingly, in the case where the dynamic range of the wireless signal received by the relay apparatus is 32 dB, the difference between the level of the signal component 1001b and the level of the signal component 1001a may be 32 dB at the maximum depending on the positional relationship between the first wireless communication terminal and the second wireless communication terminal. In this case, provided that the leakage ratio to the adjacent channel is −25 dB, the D/U ratio 1010 is −7 dB. Thus, the required D/U ratio of 22 dB cannot be fulfilled.
As described above, in the case where a plurality of communication terminals perform communication using two adjacent channels, normal communication may not be possible due to jamming from a signal of another channel, even if the wireless signal received by the relay apparatus has the quality in conformity to the Standard and the level of the wireless signal is within the dynamic range regulated by the Standard. In such a case, the first and second problems can be solved, but the third problem of fulfilling the predetermined D/U ratio cannot be solved.
Next, a case where communication is jammed by a signal leaking from the second adjacent channel will be discussed. FIG. 46 is a diagram showing spectra of wireless LAN signals which are transmitted from first and third wireless communication terminals which use two channels that are away from each other by a two-channel distance.
The solid line represents the spectrum of signal a, and the dashed line represents the spectrum of signal c. Signal a transmitted from the first wireless communication terminal and signal c transmitted from the third wireless communication terminal belong to channels which are away from each other by a two-channel distance. Hereinafter, a case where signal a is jammed by a signal component leaking from signal c will be described.
Signal a has the signal component 1001a, the signal leakage component 1002a, and the signal leakage component 1003a. The signal components of signal a shown in FIG. 46 are the same as the signal components of signal a shown in FIG. 45 and the descriptions thereof will be omitted.
Signal c has a signal component 1001c, a signal leakage component 1002c, and a signal leakage component 1003c. The signal component 1001c is a component of signal c which is to be received by the relay apparatus. The signal leakage component 1002c is a component leaking to the adjacent channel to the signal component 1001c. The signal leakage component 1003c is a component leaking to the second adjacent channel to the signal component 1001c. 
A D/U ratio 1010 is a difference between the level of the signal component 1001a and the level of the signal leakage component 1003c. 
In the case where the dynamic range of the wireless signal received by the relay apparatus is 32 dB, the difference between the level of the signal component 1001c and the level of the signal component 1001a may be 32 dB at the maximum depending on the positional relationship between the first wireless communication terminal and the third wireless communication terminal. In this case, provided that the leakage ratio to the second adjacent channel is −40 dB, the D/U ratio 1010 is 8 dB. Thus, the required D/U ratio of 22 dB cannot be fulfilled.
As described above, in the case where a plurality of communication terminals perform communication using two channels which are away from each other by a two-channel distance, normal communication may not be possible due to jamming from a signal of another channel, even if the wireless signal received by the relay apparatus has the quality in conformity to the Standard and the level of the wireless signal is within the dynamic range regulated by the Standard. In such a case, the first and second problems can be solved, but the third problem of fulfilling the predetermined D/U ratio cannot be solved.